Energy-saving hydrogen production by seawater electrolysis coupling tip-enhanced electric field promoted electrocatalytic sulfion oxidation

Hydrogen production by seawater electrolysis is significantly hindered by high energy costs and undesirable detrimental chlorine chemistry in seawater. In this work, energy-saving hydrogen production is reported by chlorine-free seawater splitting coupling tip-enhanced electric field promoted electrocatalytic sulfion oxidation reaction. We present a bifunctional needle-like Co3S4 catalyst grown on nickel foam with a unique tip structure that enhances the kinetic rate by improving the current density in the tip region. The assembled hybrid seawater electrolyzer combines thermodynamically favorable sulfion oxidation and cathodic seawater reduction can enable sustainable hydrogen production at a current density of 100 mA cm−2 for up to 504 h. The hybrid seawater electrolyzer has the potential for scale-up industrial implementation of hydrogen production by seawater electrolysis, which is promising to achieve high economic efficiency and environmental remediation.

performance by needle-like structure design of catalyst.Is the needle-like structure design of the catalyst beneficial for HER performance?Please add more discussions about it.4. The annotation text and scale in the figure should be clear.For example, Fig. 3d, Fig S19b, and Fig. 23.  5.The authors are suggested to explain why there appears fluctuations in durability test of the HSE and ASE systems in Figure 4d; 6.As SOR can proceeds in different way, one may be curious why electrochemical reaction (S2--2e-→ S) or products (S) are dominated in the present work, and how the authors regulated no further oxidation happens on S.
Reviewer #3 (Remarks to the Author): In this manuscript, the authors design a needle-like Co3S4 bifunctional catalyst grown on nickel foam (NF) with a unique tip structure (n-Co3S4@NF), which can achieve energy-saving hydrogen production, sulfion-rich wastewater purification, and sulfur recovery.Both experiments and calculations have been applied to explain the performance.This study is novel and very interesting.The data and methodology are reliable and the manuscript is also well organized and presented clearly.The conclusion is also clear and reliable.This manuscript can be considered for publication after addressing the following minor issues.
1.The HSE system rate by the 1.0 V commercial solar cell should provide a video.
2. There are some typos in the manuscript, which should be corrected.For example, "but is far higher than that of n-Co(OH)2@NF and r-Co(OH)2@NF"; in Figure 3e and Figure 3i, "vs.RHE".
3. Figure 4g is slightly blurry and should be improved appropriately.
4. The electro-oxidation products (elemental sulfur) of the sulfur oxidation reaction are prone to passivating the electrode surface and increasing the reaction overpotential, even making continuous operation infeasible.Can the design of the needle structure weaken this passivation effect?

Response to the Reviewer #1 General Comments:
The authors reported the coupling of hydrogen evolution reaction and the sulfion oxidation reaction toward hydrogen production from seawater.Needle-like Co3S4 catalyst grown on nickel foam was employed for the SOR and the HER.The local field enhancement at the tip of the needle-like structure was regarded as the major reason for the enhanced electrocatalytic performance.I suggest it could be rejected due to the lack of novelty and the inadequate evidence.
Response.Thanks for your helpful comments on our manuscript.Concerning your important concerns and advice, we have added experiments and characterizations to address them as much as possible.After careful checking and consideration, we have made substantial revisions to the revised manuscript (MS) and supplementary information (SI).Now we believe that the main results of this study are more clear and reliable.Below are the point-to-point responses to your comments.

Original comment 1-1:
More experimental evidences for the enhanced local electric field should be provided, as it is the primary argument of this work.The finite element numerical calculation alone is not adequate.Besides, details for the finite element numerical calculation should be provided in the experimental section.
Response 1-1.Thank you for your suggestion.We have provided more experimental evidence for the enhanced local electric field, including S 2-adsorption tests and Kelvin probe force microscopy (KPFM) tests.These tests and finite element simulations corroborate the contributions from a strong electric field at the tip.Besides, details for the finite element numerical calculation have been provided in the experimental section of the revised SI.Specifically, we experimentally evaluate the effect of tip structure in shaping the local environments.We performed the S 2-absorbing test by measuring the concentration of adsorbed S 2-on the electrodes (Fig. R1).The UV-vis absorption peak of S 2-is located at the wavelength of 230 nm.The S 2-absorption test shows that the number of S 2-adsorbed by the n-Co3S4@NF electrode is more than twice that of the r-Co3S4@NF electrode.The electrochemically active surface area (ECSA) was found to have a positive relationship with the double layer capacity (Cdl), but the Cdl value of the n-Co3S4@NF electrode is less than twice that of the r-Co3S4@NF electrode (Figs.R2   and R3).This indicates that the n-Co3S4@NF electrode has a larger electric-fieldinduced locally absorbed S 2-concentration (Nature, 2016, 537, 382-386.).This absorbing test result is consistent with the simulation results.
The electrical properties of as-prepared catalysts were further evaluated by KPFM (Nature, 2016, 537, 382-386.).The results also confirmed that the electric field is higher for the n-Co3S4@NF with needle-like structure than the r-Co3S4@NF with rod-like structure (Fig. R4).These results are consistent with the simulation results.
These experimental evidences indicate that the tip structure plays a crucial role in improving the attraction and mass transfer rate of ions in this region.It is predicted that the Co3S4 with a needle-like structure can exhibit excellent SOR activity.
The added figures and discussion have been provided in Supplementary Figs. 12 and 14-16 in the revised SI and MS, respectively.The details for the finite element numerical calculation have been provided in the experimental section of the revised SI.
The added figures (see Figs. R1-4), discussion in the revised MS and details for the finite element numerical calculation in the revised SI are also provided as follows for you to review.
"Kelvin probe atomic force microscopy experimentally confirmed that the electric field is higher for the n-Co3S4@NF with needle-like structure than the r-Co3S4@NF with rod-like structure (Supplementary Fig. 12).We then experimentally evaluate the effect of tip structure in shaping the local environments.We performed the S 2-absorbing test by measuring the concentration of adsorbed S 2-on the electrode (Supplementary Fig. 14).The ultraviolet and visible (UV-vis) absorption peak of S 2-is located at the wavelength of 230 nm.The S 2-absorption test shows that the number of S 2-adsorbed by the n-Co3S4@NF electrode is more than twice that of the r-Co3S4@NF electrode.The electrochemically active surface area (ECSA) was found to have a positive relationship with the double layer capacity (Cdl), but the Cdl value of the n-Co3S4@NF electrode is less than twice that of the r-Co3S4@NF electrode (Supplementary Figs. 15 and 16). 26,29This indicates that the n-Co3S4@NF electrode has a larger electric-field-induced locally absorbed S 2-concentration. 17

Finite element analysis (FEA)
We used the COMSOL Multiphysics solver to simulate the current density distribution and the S 2-concentration distribution under the preset electric field in the electrolyte around the surface of different catalysts, which is simplified in the conical shape to investigate the local situations. 3The dimensions of these systems are set at the scale of ten nanometers and one micrometer.
The current density distribution is simulated by the steady state research in the 'primary current distribution' module.The electrolyte conductivity was assumed to be 5 S m -1 .
The potential losses due to electrode kinetics and mass transport are assumed to be negligible, and ohmic losses are governed by the current distribution in the cell.The current density was computed using the equation:   The electrical properties of our as-prepared catalysts were further evaluated by Kelvin probe force microscopy (KPFM) and Electrostatic force microscopy (EFM).In Line 277: "As shown in Fig. 3f, the zeta potential of n-Co3S4@NF (-24.5 mV) compared to n-Co(OH)2@NF (30.4 mV) is shifted negatively.This suggests that the n-Co3S4@NF catalyst provides a weaker binding energy to Cl -and repels it.(Adv.Funct. Mater.33, 2212183 (2022))" The relevant content in the revised MS is also provided as follows for you to review."Given the local field enhancement effect of needle-like structures in improving the performance of electrocatalytic reactions, we attempt to apply this structure to the electrocatalytic SOR process. 16,17,18It can enhance the adsorption of surrounding reactive ions (e.g., S 2-and OH -), further enhancing the SOR and HER catalytic activities. 16It can be noticed that the current density in the sharpest region of the tip of the needle-like structure increases significantly, which is accompanied by enhanced electric field line densities in the localized region (Fig. 1g, h).The essence of this phenomenon is electrostatic repulsion, where free electrons migrate to the sharpest regions on the charged metal electrodes, which increases the free electron density and further leads to a climb in the local electric field strength. 17As shown in Fig. 3f, the zeta potential of n-Co3S4@NF (-24.5 mV) compared to n-Co(OH)2@NF (30.4 mV) is shifted negatively.This suggests that the n-Co3S4@NF catalyst provides a weaker binding energy to Cl -and repels it. 15" (Page 3, 4, 5, 8 and 9 in revised MS) Original comment 1-3: The analysis on the S 2p XPS spectra is not reasonable (line 138-141).In figure S9b and S10b, the peak at around 169 eV indicates the presence of SO4 2-species.Also, it is not necessary to include Co-S peaks in the S 2p spectra.They are generally included in the S 2p1/2 and S 2p3/2 peaks.
Response 1-3.Thank you very much for pointing out these issues.The related XPS spectra have been re-deconvoluted (see Figs. R5-7).
The corresponding S 2p XPS spectra have been provided in Supplementary Figs.
9b and 10b in the revised SI.
The revised figures (see Figs. R5 and R6) and discussion in the revised MS are also provided as follows for you to review.
Response 1-4.Thank you for your question.The related XPS spectra have been redeconvoluted.The peak shift of n-Co3S4@NF is 0.29 eV.The peak shift of r-Co3S4@NF is 0.02 eV.The difference in binding energy shift may be due to significant differences in sample morphology.The similar phenomenon has also been reported in previous work (Angew.Chem.Int.Ed., 2016, 55, 9548-9552).
The Co 2p XPS spectra have been provided in Supplementary Figs.9a and 10a in the revised SI, respectively.Original comment 1-5: The adsorption of reactants such as OH -, S 2-, Cl -on catalyst surface is not solely determined by the electrostatic interaction.The zeta potential values cannot be used as the indicator for the interaction between Cl -and catalyst (line 276).
Response 1-5.Thank you very much for pointing out this issue.The corrosive nature of seawater owing to the presence of Cl -poses a greater challenge to the stability of electrocatalysts for hydrogen production by seawater electrolysis.We have provided more experimental evidences for the n-Co3S4@NF's resistance to the Cl -corrosion, including open circuit potential (OCP) test (Fig. R9), Tafel plot test (Fig. R10), LSV curves before and after the CP test (Fig. R11), and ICP-MS test for the dissolution rate of Co element in the electrolyte of 1 M NaOH seawater (Fig. R12).These results, along with zeta potential (Fig. R13), SEM (Fig. R14), and three-dimensional X-ray tomography (Fig. R15) results prove that the n-Co3S4@NF is suitable for long-term electrocatalytic HER due to its resistance to the Cl -corrosion.
The details for the added discussion and related figures have been provided in the revised MS and SI (Fig. 3f, Supplementary Figs.20 and 28-32).
The related figures (see Figs. R9-15) and discussion in the revised MS are also provided as follows for you to review.
"As shown in Fig. 3f, the zeta potential of n-Co3S4@NF (-24.5 mV) compared to n-Co(OH)2@NF (30.4 mV) is shifted negatively.This suggests that the n-Co3S4@NF catalyst provides a weaker binding energy to Cl -and repels it. 15Tafel plots show the corrosion potential of n-Co3S4@NF is more positive than that of Ni foam in a 1 M NaOH seawater electrolyte (Supplementary Fig. 28), indicating a lower corrosion tendency and the improved corrosion resistance of the n-Co3S4 electrocatalyst compared to a blank Ni foam substrate. 33By testing the change of open circuit potential (OCP) along the CP test, the n-Co3S4 with high corrosion resistance shows negligible degradation over time (Supplementary Fig. 29).LSV curves before and after the CP test show a small degree of degradation and confirm the durability of the n-Co3S4@NF (Supplementary Fig. 30).As shown in Supplementary Fig. 31, the dissolution rate of Co element in the electrolyte of 1 M NaOH seawater from the n-Co3S4@NF is only about 10-20 uM, which is quite negligible.Obviously, the n-Co3S4@NF is suitable for long-term electrocatalytic HER due to its resistance to Cl -corrosion.Moreover, the Faraday efficiency of n-Co3S4@NF up to 99.1% reveals that no additional side reactions occur during hydrogen production in alkaline seawater, highlighting the high efficiency of the catalyst in HER. (Fig. 3h).Its backbone structure also retained substantial integrity upon the durability test, also indicating that the n-Co3S4@NF catalyst has excellent resistance to seawater corrosion (Supplementary Fig. 32)." "Simultaneously, we examined the crystal structures, morphology, and surface chemical states of the n-Co3S4@NF catalyst after the HER stability test in the alkaline seawater electrolyte (Supplementary Figs.19-22).The n-Co3S4@NF catalyst showed negligible changes, presenting the vast potential of the n-Co3S4@NF catalyst for hydrogen production by seawater electrolysis."(Pages 9 and 10 in revised MS)      Fig. R19 | Durability measurement of n-Co3S4@NF for removing H2S in industrial syngas via 1 M NaOH solution with 2% H2S/syngas.The measurement was carried out at a galvanostatic current of 100 mA cm -2 , and the corresponding potential of n-Co3S4@NF was maintained at around 0.8 V for 120 h.The fresh electrolytes were changed every 24 h.

Response to the Reviewer #2
General Comments: This manuscript fabricated a bifunctional n-Co3S4@NF catalyst with a needle-like structure for SOR-assisted energy-saving hydrogen production by seawater electrolysis.
The HSE system assembled by the bifunctional n-Co3S4@NF is not only able to reduce power consumption up to 67.9% compared to the conventional ASE system but can S also be grid-connected to a thermoelectric/photovoltaic power generation system for efficient and long-lasting hydrogen production.The results from multiple characterizations and calculations are reliable.Besides, the plots and figures are welldesigned and shown in a logical manner.Overall, this work is interesting and this manuscript is well organized, I would be pleased to recommend that it be published after minor revision.Detailed comments are listed as follows: Response.Thank you for your very positive comment on our work and your recommendation for publication after revision.

Original comment 2-1:
Regarding the DFT calculation, structural models of reaction intermediates adsorbed on the catalyst should be provided.
Response 2-1.Thank you for your suggestion.Structural models of reaction intermediates adsorbed on the catalyst have been provided in Supplementary Fig. S34 in the revised SI.The figures are also provided as follows for you to review (see Fig. R20).

Original comment 2-2:
The details for Comsol simulations should be provided.

Original comment 3-4:
The electro-oxidation products (elemental sulfur) of the sulfur oxidation reaction are prone to passivating the electrode surface and increasing the reaction overpotential, even making continuous operation infeasible.Can the design of the needle structure weaken this passivation effect?
Response 3-4.Thank you for your question.We believe that the design of the needle structure plays a role in weakening this passivation effect.First, from a geometric perspective, the needle structure has a smaller scale, making it difficult for sulfur to adsorb on the catalyst surface or more easily desorb, thereby avoiding the formation of a thick sulfur adsorption layer.Second, materials with needle structures have a larger electrochemical active area, and the generated sulfur does not easily cover all surfaces.

REVIEWERS' COMMENTS
Reviewer #2 (Remarks to the Author): The authros did a good job to improve the manuscript, and all the concerns raised were well addressed, the revision is thus recommended to be accepted for Nat Commun Publication as it was.
Reviewer #3 (Remarks to the Author): The revisions on the manuscript is sound and I am happy to suggest its acceptance in Nat Commun.
During the proofing process, it would be better if more classical or timing reports on related topic could be cited, and the English writing could be further improved.
This absorbing test result is consistent with the simulation results.It indicates that the tip structure plays a crucial role in improving the attraction and mass transfer rate of ions in this region.It is predicted that the Co3S4 with a needle-like structure can exhibit excellent SOR activity."(Page 2 and 5 in revised MS) "Kelvin probe force microscopy (KPFM) measurement KPFM measurement is a powerful technique for simultaneous obtaining the topography and surface potential with nanometer scale spatial resolution and millivolt sensitivity in potential resolution via a dual pass process.The KPFM measurement was implemented based on an AFM (NT-MDT NTEGRA) system using a conductive NSG30/Pt tip.All the images were recorded in semi contact mode at room temperature with a relative humidity of 30%.Si cantilevers (NT-MDT with a typical curvature radius of the tip of 35 nm and a typical resonant frequency of 0.5 kHz) were used.In a standard KPFM procedure, a DC bias voltage is continually adjusted with a potential feedback loop to nullify the surface potential difference between the probe and sample.During the SemiContact 2-pass KPFM measurement, an AC bias voltage, superimposed on a DC bias voltage, is applied between the probe and sample.Thus, the potential difference can be written as V=Vdc -∆V + Vac sin(ωt) where Vdc and Vac are DC bias and AC bias, respectively.The Δ = ( -)/ is the contact potential difference (CPD) between the probe and sample.

Fig
Fig.R2| The CV curves of n-Co3S4@NF and r-Co3S4@NF under different scan rates in the non-Faraday region.
Fig. b-c and g-h, the n-Co 3 S 4 @NF and r-Co 3 S 4 @NF shows a negative CPD against the substrate, which reveals that samples had a negative charge relative to the substrate.(Chemical Engineering Journal, 2021, 418, 129422) It was revealed by KPFM that the electric field at the n-Co 3 S 4 @NF tip was larger than that of r-Co 3 S 4 @NF (Figure SX).On the other hand, the surface potential difference value The citations need improvement.More relevant reference should be cited at proper place such as line 100, line 150, line 155, line 159, line 277… Also, are there any reference in which the local electric field and the ion concentration are analyzed by the finite element numerical calculation?If yes, relevant reference should be cited.Response 1-2.Thank you very much for pointing out these issues.We have made appropriate corrections and highlighted the contents in yellow in the revised MS.Some references (including (Energy Environ.Sci. 16, 285-294 (2023), Nature 537, 382-386 (2016), Adv.Mater.33, e2007377 (2021)) which have analyzed the local electric field and the ion concentration by the finite element numerical calculation have been cited in the revised MS.Line 100: "Given the local field enhancement effect of needle-like structures in improving the performance of electrocatalytic reactions, we attempt to apply this structure to the electrocatalytic SOR process (Energy Environ.Sci. 16, 285-294 (2023), Nature 537, 382-386 (2016), Adv.Mater.33, e2007377 (2021))" Line 150: "It can enhance the adsorption of surrounding reactive ions (e.g., S 2- and OH -), further enhancing the SOR and HER catalytic activities.(Energy Environ.Sci. 16, 285-294 (2023)" Line 159: "The essence of this phenomenon is electrostatic repulsion, where free electrons migrate to the sharpest regions on the charged metal electrodes, which increases the free electron density and further leads to a climb in the local electric field strength.(Nature 537, 382-386 (2016))"

Fig. R9 |
Fig. R9 | Open circuit potential (OCP) of n-Co3S4@NF before and after 10 h stability test in 1 M NaOH seawater at a current density of 100 mA cm -2 .

Fig. R12 |
Fig. R12 | Atomic concentrations of Co in the electrolyte during the chronopotentiometry test at the current density of 100 mA cm -2 for 150 h.

Fig. R15 |
Fig. R15 | Three-dimensional X-ray tomography images of n-Co3S4@NF electrodes (a) before and (b) after 210 h HER stability test at the current density of 100 mA cm -2 in 1 M NaOH seawater.
Fig. R16 | A demo for the n-Co3S4@NF used as SOR electrocatalyst for the purification of realistic sulfion-containing sewage form a natural gas field.(a) The optical photograph of the sulfion-containing sewage collected from Dazhou, Sichuan Province, PRC.(b) UV-vis spectra of different solutions.(c) The LSV curves for the SOR over n-Co3S4@NF in different reaction solutions, including the collected realistic sulfion-containing sewage and 1 M Na2S + 1 M NaOH as a blank control.

Response 2- 2 .
Fig. R21 | A rough techno-economic analysis of the HSE and ASE systems for hydrogen production.